Capacitor

ABSTRACT

A capacitor including a cathode of a porous coating including an oxide of at least one metal selected from the group consisting of ruthenium, iridium, nickel, rhodium, platinum, palladium, and osmium disposed on each of two opposed electrically conducting plates; an anode disposed between and spaced from the porous coatings and the electrically conducting plates and including a metal selected from the group consisting of tantalum, aluminum, niobium, zirconium, and titanium; and an electrolyte disposed between and in contact with the porous coatings and the anode.

This disclosure is a division of patent application Ser. No. 08/514,145,filed Aug. 11, 1995, now U.S. Pat. No. 5,559,667, which is a division ofpatent application Ser. No. 08/282,229, filed Jul. 29, 1994, now U.S.Pat. No. 5,469,325, which is a continuation-in-part of patentapplication Ser. No. 08/035,224, filed Mar. 22, 1993, now U.S. Pat. No.5,369,547, the disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention concerns improved capacitors and particularlycapacitors employing a pseudo-capacitor-type cathode and a wet slug-typecapacitor anode to achieve improved performance including increasedenergy storage density.

BACKGROUND OF THE INVENTION

Tantalum wet slug capacitors have long been known in the capacitor arts.An example of the structure of a wet slug tantalum capacitor isdescribed in U.S. Pat. No. 4,780,797. Fundamentally, as described there,the wet slug capacitor includes a tantalum or tantalum-plated containerthat is the cathode or negative terminal of the electrolytic capacitor.An electrolyte and a porous sintered tantalum anode are disposed withinthe container. Tantalum forms a native oxide on exposed surfaces thatmay be increased in thickness by anodic oxidation. In the conventionalwet slug capacitor, both the anode and cathode have insulating tantalumoxide coatings that are spaced apart from each other but are both incontact with the electrolyte, typically a sulfuric acid solution. Sincesulfuric acid is electrically conductive, aconductor-insulator-conductor structure including metal, oxide coating,and electrolyte is present at both the anode and the cathode. Each ofthese conductor-insulator-conductor structures is itself a capacitor,i.e., an anode capacitor and a cathode capacitor. The capacitances ofthese electrode capacitors are to some degree determined by thethickness of the oxide layers formed on the anode and the cathode.Increasing the thickness of the anode oxide layer but not the cathodeoxide layer, for example, by anodic oxidation, increases the breakdownvoltage that a wet slug capacitor can withstand but reduces the overallcapacitance of the capacitor. Typical breakdown voltages for a singlecapacitor can range from ten to one hundred twenty-five volts.

In the wet slug capacitor, the anode capacitance is effectivelyelectrically connected in series with the cathode capacitance. As iswell known, the net capacitance of two capacitors connected in series issmaller than the smaller of the capacitances of the two capacitors.Because the oxide layer at the anode of a wet slug capacitor is usuallymuch thicker than the thickness of the oxide layer at the cathode, theanode capacitance of a wet slug capacitor is smaller than the cathodecapacitance. For example, in a typical structure, the anode capacitancemay be 3,100 microfarads and the cathode capacitance may be 8,700microfarads. The resulting net capacitance of that capacitor is about2,300 microfarads.

Although wet slug capacitors having useful capacitances and breakdownvoltages can be readily produced, there is always a desire to increasethe capacitance per unit volume of those capacitors, i.e., the energystorage density, without a reduction in the breakdown voltage. Oneproposed method of increasing the energy storage density of a wet slugcapacitor is described in the cited patent. In that patent, a number ofmetallic films are deposited on the inside of the container of thecapacitor. In particular, it is suggested that a film selected from theplatinum group of metals, i.e., ruthenium, rhodium, palladium, andplatinum, be alloyed with the tantalum of the container in segregatedislands where the native oxide has been removed from the tantalum.Various techniques can be employed to deposit the platinum group metal,such as sputtering and electrolytic or electroless plating, followed bya heat treatment at a relatively high temperature, for example, from925° C. to 1,500° C. Preferably, a platinum group metal layer issubsequently deposited on the islands to form a spongy layer. Theplatinum group metals apparently improve the energy storage density ofcapacitors having the structure described in the patent.

In U.S. Pat. No. 4,942,500, a platinum group metal is applied to acapacitor cathode by cladding, i.e., by rolling a very thin layer of theplatinum group metal with the tantalum metal. Explosive bonding is alsomentioned. In U.S. Pat. No. 5,043,847, electrolytic co-deposition of abase metal and platinum group metal on the inside surface of a wet slugcapacitor container is described. Addition of the platinum group metalby these techniques is said to increase the energy storage density.

A different type of electrolytic capacitor, frequently referred to as anelectrochemical capacitor, employs so-called pseudocapacitiveelectrodes. These capacitors generally have metal oxide electrodesincluding a substrate of titanium or tantalum. Typically, a hydratedchloride of the metal, which may be ruthenium, is dissolved in isopropylalcohol and applied to a heated titanium or tantalum substrate. The heatdrives off the solvent, resulting in the deposition of a metal chloride.That chloride is heated to a high temperature in air to convert themetal chloride to an oxide. For example, the metal chloride film may beheated to about 250° C. for approximately one-half hour to completelyremove the solvent and to drive off water. Thereafter, in a secondelevated temperature step, for example, at approximately 300° C., a highsurface area film of the oxide of the metal, for example, rutheniumoxide, is formed on the substrate. The oxide film is highly porous,meaning that it has a very high surface area. An electrochemicalcapacitor includes such electrodes as the anode and as the cathode,typically with a sulfuric acid solution electrolyte. The electricalcharge storage mechanism is not yet fully understood. Electrical chargesmay be stored on the very large surface areas of the two electrodes,providing the capacitance characteristic. Electrical charges may bestored by a reversible change in the oxidation state of a material in anelectrode. No matter what the charge storage mechanism is, it issubstantially different from the charge storage mechanism of a wet slugcapacitor electrode.

Although electrochemical capacitors can provide much higher energystorage densities than wet slug capacitors, the breakdown voltage ofindividual cell electrochemical capacitors is very low, typically onlyabout one volt, i.e., essentially the dielectric breakdown voltage ofthe electrolyte. Even if electrochemical capacitors are connected inseries, it is difficult to produce a practical capacitor with abreakdown voltage comparable to the breakdown voltages of wet slugcapacitors. Thus, electrochemical capacitors have not found wide usage.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved capacitor havinga practical breakdown voltage and a high energy storage density.

According to one aspect of the invention, a capacitor comprises acathode including a porous coating of an oxide of at least one metalselected from the group consisting of ruthenium, iridium, nickel,rhodium, platinum, palladium, and osmium, an anode spaced from theporous coating and including a metal selected from the group consistingof tantalum, aluminum, niobium, zirconium, and titanium and spaced fromthe porous coating, and an electrolyte disposed between and in contactwith the porous coating and the anode.

According to another aspect of the invention, a capacitor comprises afirst metal body, a cathode comprising a porous coating including anoxide of at least one metal selected from the group consisting ofruthenium, iridium, nickel, rhodium, platinum, palladium, and osmiumdisposed on the first metal body, an anode including a metal selectedfrom the group consisting of tantalum, aluminum, niobium, zirconium, andtitanium disposed on a second metal body opposite and spaced from thefirst metal body, and an electrolyte disposed between and in contactwith the porous coating and the anode.

According to yet another aspect of the invention, a capacitor includes aplurality of capacitor cells, each cell including a first metal bodyhaving opposed first and second surfaces; a cathode comprising a porouscoating including an oxide of at least one metal selected from the groupconsisting of ruthenium, iridium, nickel, rhodium, platinum, palladium,and osmium disposed on the first surface of said first metal body; ananode including a metal selected from the group consisting of tantalum,aluminum, niobium, zirconium, and titanium disposed on the secondsurface of the first metal body; an electrolyte in contact with thecathode opposite the first metal body wherein the plurality of thecapacitor cells are disposed in a serial arrangement, the electrolyte ofone cell contacting the second surface of each first metal body and afirst surface of the first metal body of the next adjacent cell; asecond metal body having first and second opposed surfaces disposed atone end of the serial arrangement and including a cathode comprising aporous coating including an oxide of at least one metal selected fromthe group consisting of ruthenium, iridium, nickel, rhodium, platinum,palladium, and osmium disposed on one side of the second metal body andopposite an anode of a first metal body in the serial arrangement, butno anode, and functioning as a cathode of the capacitor and anelectrolyte disposed between and contacting the porous coating of thesecond metal body and the anode of the opposite first metal body in theserial arrangement; and a third metal body having first and secondopposed surfaces and disposed at the other end of the serial arrangementand including an anode comprising a metal selected from the groupconsisting of tantalum, aluminum, niobium, zirconium, and titaniumdisposed on one side of the third metal body and opposite a porouscoating of a first metal body in the serial arrangement, but no porouscoating, and functioning as an anode of the capacitor and an electrolytedisposed between and contacting the anode of the third metal body andthe porous coating of the opposite first metal body in the serialarrangement.

In the invention, one electrode of a capacitor is an wet slugcapacitor-type electrode, for example, the anode. The other electrode isan electro-chemical-type capacitor electrode employing a porous coatingincluding an oxide of at least one metal selected from the groupconsisting of ruthenium, iridium, nickel, rhodium, platinum, palladium,and osmium. For the same capacitance value, the cathode of a capacitoraccording to the invention is reduced in size compared to a conventionalwet slug capacitor electrode. If the volume of the conventional wet slugcapacitor for a particular capacitance is maintained, then the anode ofa capacitor according to the invention can be increased in size relativeto the conventional wet slug capacitor anode, increasing the capacitanceand the energy storage density as compared to a conventional wet slugcapacitor. In addition, if the capacitance of a conventional wet slugcapacitor is maintained, then the volume of a corresponding capacitoraccording to the invention can be made smaller than the conventional wetslug capacitor, increasing energy storage density. A high breakdownvoltage, characteristic of the conventional wet slug capacitor, isobtained in the invention because of the presence of the conventionalwet slug capacitor anode while realizing increased energy storagedensity because of the presence of the pseudocapacitor cathode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of a capacitor according to an embodiment ofthe invention;

FIG. 2 is a cross-sectional view of an alternative embodiment of acapacitor according to the invention;

FIG. 3 is a cross-sectional view of a single cell prismatic capacitoraccording to an embodiment the invention;

FIG. 4 is a cross-sectional view of a multiple cell prismatic capacitoraccording to an embodiment of the invention;

FIG. 5 is a cross-sectional view of an alternative embodiment of acapacitor according to the invention;

FIG. 6 is a cross-sectional view of a single cell prismatic capacitoraccording to an embodiment the invention;

FIG. 7 is a cross-sectional view of a multiple cell prismatic capacitoraccording to an embodiment of the invention;

FIG. 8 is a cross-sectional view of an alternative embodiment of acapacitor according to the invention; and

FIG. 9 is a cross-sectional view of a multiple cell prismatic capacitoraccording to an embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is an exploded view of an embodiment of the invention and FIG. 2is a cross-sectional view of another embodiment of the invention. Likereference numerals are used in those and all other figures to designatethe same elements.

In FIG. 1, a capacitor according to the invention includes a metalcontainer 1, typically a tantalum container. However, metals other thantantalum may be used in embodiments of the invention. Typically, thecontainer 1 is the cathode of the capacitor and includes a lead 2 thatis welded to the container. An end seal, a cap 3, includes a second lead4 that is electrically insulated from the remainder of the cap by afeedthrough 5 seen in FIG. 2. In the assembled capacitor, the cap 3 isbonded to the container 1 by conventional means, for example, bywelding. The insulating feedthrough 5 of the lead 4 is likewiseconventional and may include a glass-to-metal seal through which thelead 4 passes. A conventional porous sintered tantalum anode 6 with ananodic oxide film coating is electrically connected to the lead 4 anddisposed within the container 1. Direct contact between the container 1and the anode 6 is prevented by electrically insulating spacers 7 and 8within the container 1 that receive the ends of the anode 6. Theretaining insulators 7 and 8 are conventional.

In the embodiment of the invention shown in FIG. 1, a metal body 11,such as a metal foil, is disposed within and is in electricalcommunication with the metal container 1. The communication may beestablished, for example, by welding the metal body to the insidesurface of the metal container 1. The inside surface of the metal body11 includes a porous coating 12 including a metal oxide. The porouscoating preferably includes an oxide of a first metal. The first metalis selected from the transition metals in Group VIII of the PeriodicTable of Elements that have at least two stable oxidation states in theelectrolyte used in the capacitor. The metal is particularly selectedfrom the group consisting of ruthenium, iridium, nickel, rhodium,platinum, palladium, and osmium. The porous coating may also include anoxide of a second metal selected from the group consisting of tantalum,titanium, and zirconium. The second metal oxide is not believed to beelectrically active but increases the surface area of the porous coatingand/or extends the mixture used to form the porous coating. The secondmetal oxide component is not essential in the capacitor cathode. In apreferred embodiment of the invention, the porous coating includesoxides of ruthenium and tantalum.

In the embodiment of the invention shown in FIG. 2, a porous coating 13,i.e., the same as the porous coating 12 of the embodiment of FIG. 1, isformed directly on the inside surface of the metal container 1. Themetal body 11 employed in the embodiment of the invention shown in FIG.1 is thus eliminated, reducing costs.

In the capacitors of FIGS. 1 and 2, each capacitor includes twoelectrodes. One of the electrodes, the anode 6, is preferably aconventional sintered porous tantalum anode with an oxide film coatingof the type used in conventional wet slug tantalum capacitors. Inaddition, the anode may be made of another one of the so-called valvemetals, i.e. aluminum, niobium, zirconium, and titanium. The otherelectrode includes the metal container 1, the metal body 11 with theporous coating 12 or the porous coating 13 on the container 1 and issimilar to one of the electrodes used in a pseudocapacitor. As a resultof that combination, advantages of a pseudocapacitor and of a wet slugcapacitor are achieved without the disadvantages of either of thoseknown capacitor structures. The cathode capacitance is greatly increasedover the cathode capacitance of a conventional wet slug capacitorbecause of the very large surface area and the very small effective"plate separation". (Plate separation refers to modeling of the cathodeas a theoretical parallel plate capacitor with two plates having areas Aseparated from each other by a distance d.) Because of the increasedcapacitance contributed by the pseudocapacitor cathode for a particularvolume, the cathode can be reduced in size, providing space for an anodeof increased size, larger than the wet slug capacitor anode of aconventional wet slug capacitor having the same capacitance.Alternatively, for the same volume as a conventional wet slug capacitor,a much larger capacitance can be achieved.

As already described with respect to one example of a conventional wetslug capacitor, a sintered anodic oxide coated tantalum anode has acapacitance of 3,100 microfarads. A pseudocapacitor cathode replacingthe cathode of the conventional tantalum capacitor (having a capacitanceof 8,700 microfarads) has a capacitance of 0.2 farads. Since, as in theconventional apparatus, these electrode capacitances are electricallyconnected in series, in the capacitor according to the invention, theoverall capacitance is calculated as 3,050 microfarads, an increase incapacitance of one-third over the conventional wet slug capacitor.Measured capacitances of capacitors in accordance with the inventionconfirm the accuracy of this calculation.

When a voltage is applied to a capacitor according to the invention, thevoltage is divided across the oxide film coating the anode and thepseudocapacitor cathode. Because the capacitance of the anode is muchsmaller than the capacitance of the pseudocapacitor cathode, the voltageapplied to the capacitor naturally divides unequally across theelectrodes. A large proportion of the applied voltage appears across theanode oxide film and not across the electrolyte. A much smallerproportion of the applied voltage appears across the pseudocapacitorelectrode. As a result, a capacitor according to the invention cansustain a much higher voltage, i.e, has a much higher breakdown voltage,than a conventional pseudocapacitor. In other words, increasedcapacitance as observed in a pseudocapacitor is achieved in theinvention without the disadvantage of the low breakdown voltage observedin those known capacitors. As well known in the art, the oxide filmcoating the valve metal anode, particularly a tantalum or aluminumanode, can be increased to a desired thickness, increasing the capacitorbreakdown voltage, in an anodic oxidation process.

The porous coating, whether formed on a metal body or directly on theinside surface of a metal container of a capacitor according to theinvention, is formed using conventional processes. Examples of methodsof forming such porous coatings on metal bodies are described innumerous publications. For example, the formation of similar capacitorelectrodes is described in U.S. Pat. No. 4,766,522. Electrolysis cellelectrodes including similar but very thick coatings are described insome of the examples appearing in U.S. Pat. No. 3,632,498.

In a preferred process, hydrated ruthenium chloride (RuCl₃.3H₂ O) isdissolved in isopropyl alcohol to form a solution having a concentrationof one to three percent. Preferably, an enhancing agent, such as achloride of tantalum, is added to the solution. A mixture having anatomic ratio of about one ruthenium atom to three tantalum atomsproduces a higher capacitance film than do mixtures with differentratios of tantalum to ruthenium atoms. The rate of dissolution of thechlorides in alcohol can be increased by the addition of about 10milliliters of hydrochloric acid per 100 milliliters of isopropylalcohol. Titanium, nickel, and zirconium compounds may also be used inplace of the tantalum chloride to improve performance of the capacitorsaccording to the invention. While the preferred process employschlorides because of their solubility, other inorganic and organic saltsof the metals can also be employed in the formation of the porouscoating.

Whether the substrate on which the porous coating is disposed is a metalbody or the inside surface of a metal capacitor container, the substrateis preferably roughened before deposition of the coating to increase theadhesion of the solution subsequently applied to the substrate informing the coating. The surface may be roughened by chemical treatment,for example, with sulfuric acid, hydrochloric acid, or oxalic acid, orby a mechanical process, such as sand blasting, although mechanicalprocesses are not preferred over chemical treatments. The tantalum ortitanium substrate is then heated to about 85° C. and the solution isapplied. The elevated temperature of the substrate results in rapidevaporation of the alcohol solvent, leaving the formerly dissolvedchlorides in place as a film on the substrate.

After the formation of the metallic chloride film, the substrate isheated to a temperature of about 250° C. in air to drive off anyremaining solvent and the water contained in the hydrated chloride. Inaddition, some of the chlorine may be driven off at that time. Theheating continues in air for about one hour after which the temperatureis increased to approximately 300° C. for a time sufficient to oxidizethe metal components of the coating. For example, the oxidizingtreatment in air may continue for about two hours. The resulting coatingis insoluble in water and sulfuric acid, has pores as small as 5nanometers, and has a surface area of up to about 120 square meters pergram.

The completed capacitor includes a fluid electrolyte 14, shown in FIG.2, disposed between and in contact with both of the electrodes toprovide a current path between the electrodes 6 and 11 or 6 and 13. Thefluid electrolyte may be any of the conventional electrolytes employedin capacitors, most typically a sulfuric acid solution when the anode istantalum. In other constructions, different electrolytes are used. Forexample, when the anode is aluminum, an ammonium salt dissolved in anon-aqueous solvent, such as glycol or a glycol-like solvent, may beemployed because sulfuric acid attacks aluminum. When the cathode isnickel, then an aqueous solution of potassium hydroxide is preferredover sulfuric acid as an electrolyte. As is conventional, the materialsof construction of the capacitor that are contacted by the electrolyteare chosen to be impervious or extremely resistant to the effects of theparticular electrolyte employed.

The embodiments of the capacitor according to the invention shown inFIGS. 1 and 2 are similar in shape and arrangement to conventionaltantalum wet slug capacitors. Other embodiments of the inventionresemble the "jelly roll" structure of conventional foil capacitors. Inthat configuration, the anode slug is replaced by a conventional foil oftantalum or aluminum, or any of the other valve metals, wound in jellyroll fashion as the anode. Some decrease in capacitance is experiencedin replacement of the anode slug with the rolled foil. However, anincreased capacitance over the conventional jelly roll foil capacitorsis achieved in the invention because of the presence of the cathodeincluding the porous coating.

Capacitor cells of still different geometrical configuration accordingto embodiments of the invention can be easily made. The capacitors cellscan be interconnected in series to form a capacitor having a higherbreakdown voltage than an individual cell. An example of an embodimentof the invention including a single cell is shown in FIG. 3. A capacitoraccording to an embodiment of the invention and including a plurality ofcells arranged and interconnected serially is shown in FIG. 4.

In FIG. 3, a capacitor according to an embodiment of the inventionincludes opposed metal bodies 21 and 22, preferably thin metal plates orfoils. The plates are separated by an insulating sealant 23 that isadhered to both of the plates 21 and 22. FIG. 3 (and FIG. 4) is asectional view and the capacitor can have any desired shape in plan. Forexample, if the capacitor has a circular shape in plan view, thenpreferably the sealant 23 is a unitary, annular body adhered to bothplates, sealing and forming a sealed package. If the capacitor has othershapes in plan view, it is still preferred that the sealant 23 be aunitary body following the perimeter of the plates, i.e., the capacitor,defining a closed volume between the two plates 21 and 22. The sealantmay extend beyond the plates. The sealant may be a laminate of resinlayers that are thermally sealed to each other.

The plates 21 and 22 are preferably tantalum, although other metals,such as titanium, may be employed. A porous tantalum anode 24 or ananode of another valve metal is formed on plate 21 and disposed withinthe sealed volume defined by the plates 21 and 22 and the sealant 23.The inside surface of the plate 22 is coated with a porous coatingincluding a metal oxide prepared as described above, thereby forming apseudocapacitor cathode. In order to avoid direct contact between theanode 24 and the porous coating on the inside surface of the plate 22, aspacer is interposed between the anode 24 and the plate 22. Mostpreferably, the spacer includes a plurality of masses of an electricallyinsulating material disposed between and contacting the anode 24 and theplate 22. A fluid electrolyte 26, such as a solution of sulfuric acid,potassium hydroxide, or an ammonium salt, is present between and incontact with the anode 24 and the plate 22. The electrolyte 26 directlycontacts the spacer 25 so that the spacer material must be impervious tothe electrolyte.

In the capacitor of FIG. 3, the plate or metal body 22 has a porouscoating including a metal oxide formed on one surface in the same mannerthat the porous coating is formed on metal body 11 or on the insidesurface of the container 1 of the embodiments of the invention alreadydescribed. After the formation of that porous coating, the spacer 25 isdeposited on the porous coating. The individual spacing masses may beformed by printing, such as silk screening, while dissolved in a solventthat is subsequently removed, for example, by the application of heat,or by the deposition of individual masses of a melted electricallyinsulating material. If the electrolyte is sulfuric acid, then thespacer may be made of polyolefin, polyethylene, or polypropylene, forexample. Other kinds of spacers can be employed instead of theindividual masses illustrated in FIG. 3. For example, a glass fiberpaper, plastic fibers, or an ion-permeable material, such as NAFION, maybe inserted between the anode 24 and the plate 22 to prevent directcontact of the electrodes. NAFION is a trademark of DuPont de NemoursCo. of Wilmington, Del. for a fluoropolymer containing channels ofsulfonate groups that are permeable to cations, such as hydrogen ions.The spacing masses may be located on the anode 24 rather than on theporous coating or may not be fixed to either electrode.

The plate 21, which is preferably the same size and shape as the plate22, is masked over the area where the sealant 23 will be adhered. Plate21 may be a thin metal foil, for example, 0.001 inch (25 micrometers) inthickness. Tantalum powder held together by a binder, such as stearicacid, if needed, is applied to the plate 21 under pressure. The tantalumpowder binder is driven off, for example, by heat, and the powder issintered in an inert atmosphere to produce a high surface area porousanode. Finally, that anode is anodically oxidized to form tantalum oxideover the surface of the sintered powder to a desired thickness. Similaranodes may be made of aluminum, niobium, zirconium, and titanium.

The plates 21 and 22 are then brought together with the spacer 25preventing direct contact of the plate 22 and the anode 24. The spacermasses may be about 0.001 inch (25 micrometers) high, 0.005 inch (125micrometers) in diameter, and spaced about 0.050 inch (1.25 millimeters)apart in a regular pattern. In order to form a stable assembly, thesealant 23 is then applied at the periphery of the two plates 21 and 22to form a closed package retaining the fluid electrolyte 26. A hot meltpolyolefin or epoxy may be employed as the sealant 23. Subsequently, thesealant can be broken or opened so that the fluid electrolyte 26 can beinjected into the package. The interior of the package may be evacuatedin advance of injecting the electrolyte. After the fluid electrolyte isin place, the sealant is resealed with additional sealant material.Leads can be easily attached to the plates 21 and 22 before, during, orafter assembly of the capacitor.

A capacitor according to an embodiment of the invention and including aplurality of individual capacitor cells 30 interconnected in series isillustrated in FIG. 4. The serial arrangement of the cells 30 isterminated at opposite ends of the arrangement by plates 21 and 22,respectively. With the exception of those two plates, which areidentical to the corresponding elements of the capacitor shown in FIG.3, the remainder of the capacitor units in the capacitor of FIG. 4 areidentical cells 30. Since plate 22 on which spacers 25 are disposed andplate 21 on which the anode 24 is formed have already been described, norepetition of the description of those elements is required.

Each cell 30 includes a bipolar metal plate or metal body 31. On oneside of the metal body 31, a porous coating including a metal is formedin accordance with the preceding description. For example, a porousruthenium oxide film containing tantalum oxide may be present on oneside of the plate 31. Electrically insulating spacing masses 32 aredisposed in a pattern on that porous oxide coating. A porous tantalumanode 33, or an anode of a different material, is formed on the oppositeside of the plate 31, completing the bipolar element of the cell 30.Generally, the anode 33 is formed first while a mask is present on theopposite side of the plate 31 to prevent the formation of excessivetantalum oxide. After the anode is completed, the mask is removed andthe oxide coating is formed on the side of the plate 31 opposite theanode. Thereafter, the spacer masses 32 are formed on the oxide coating.Finally, the sealant 34 is applied to one side of the plate 31,completing the cell 30. The order of the fabrication steps can bechanged and the spacing masses can be formed on the anode rather than onthe oxide coating provided appropriate changes are made in the cells atthe ends of the serial arrangement.

The cells are then assembled by attaching the sealant to the plates ofadjacent cells. The sealant may extend beyond individual cells and maymerge into a single body along the whole length of the capacitor. Thesealant may include laminated resin layers that are heat sealed togetherbeyond the edges of the plates 31. After the serial arrangement of theunit cells is assembled, the end units, i.e., the plate 21 with theattached anode 24 and the plate 22 with the attached spacer masses 25 onthe porous coating, are applied to opposite ends of the serialarrangement of identical unit cells to complete the mechanical assemblyof the capacitor. A fluid electrolyte is added to each of the cellsthrough openings made in the sealant. The volume occupied by theelectrolyte may be evacuated before the electrolyte is introduced. Afterinjection of the electrolyte, the sealant is again closed, completingthe capacitor.

When a capacitor like the embodiment shown in FIG. 4 includes a numberof cells, it is not always possible to produce cells having identicalcharacteristics. Particularly in capacitors according to the invention,where one electrode of each cell is significantly different in one ormore of capacitance, resistance, and leakage current from the otherelectrode of that cell, excessive voltages may be applied to variouscells. In order to avoid application of excessive voltages, a resistor,such as the resistors 35 illustrated in FIG. 4, can be connected acrossthe plates of each cell. If resistors are so employed, one such resistorshould be connected across each pair of metal plates in the entireserial arrangement rather than the partial connection shown in FIG. 4which is shown only for illustrative purposes. The resistors should eachhave essentially the same resistance and provide a current path carryingsubstantially more current, for example, larger by a factor of ten, thanthe leakage current that flows through the capacitor. Although theresistors 35 are illustrated as discrete elements in FIG. 4, distributedresistors between adjacent capacitor plates can be provided by employinga sealant with a desired, finite resistivity. Alternatively, anelectrically conducting paint can be applied to the sealant in one ormore stripes interconnecting the capacitor plates, i.e., electrodes, ofthe capacitor embodiments of FIGS. 3 and 4.

FIG. 5 illustrates an alternative embodiment of a capacitor according tothe invention. The structure of FIG. 5 is identical to the structure ofFIG. 2 with the exception of the electrolyte. In the structure of FIG.2, the electrolyte 14 is a fluid. In the capacitor embodiment of FIG. 5,the electrolyte 14' is a solid electrolyte, such as polypyrrole, NAFION(an ion permeable, electron impermeable commercially availablematerial), and polyaniline, and including semi-solids, such as theaqueous electrolyte solutions already described with silica added toform a gel. Similarly, FIG. 6 shows in cross-section another capacitorembodiment according to the invention. Although a similar capacitorembodiment shown in FIG. 3 includes a fluid electrolyte 26, in theembodiment of FIG. 6, the electrolyte 26' is a solid electrolyte. Theelectrolyte 26' acts as a spacer, eliminating the need for the spacingmasses 25 employed in the embodiment of FIG. 3. In addition, thepresence of the solid electrolyte 26' eliminates the need for thesealant or container 23 separating the plates 21 and 22 and retainingthe fluid electrolyte since the solid electrolyte does not flow norevaporate. In other words, in a capacitor according to the inventionemploying a solid electrolyte, no container is necessary.

FIG. 7 illustrates a capacitor made by laminating multiple cells of thetype individually illustrated in FIG. 6 and employing a solidelectrolyte 26'. Although each capacitor cell 30 includes a sealant 34defining a container, as discussed above, a container is not necessarywhen the capacitor cell includes the solid electrolyte 26' disposedbetween each electrode pair including an anode and a cathode. The solidelectrolyte also functions as a spacer, keeping those electrodes apart,thereby preventing short-circuiting.

FIG. 8 is a sectional view of still another embodiment of the capacitoraccording to the invention. The capacitor of FIG. 8 includes opposedmetal plates or foils 22. The porous coating 24 of one or more metaloxides functioning as a cathode is disposed on one of the surfaces ofeach of the metal plates or foils 22. The porous coatings 24 face eachother. A conventional valve metal capacitor anode 6, such as anodicallyoxidized tantalum, is disposed between and spaced from the porouscoatings 24. The space between the porous coatings 24 and the anode 6 isfilled with an electrolyte. If that electrolyte is a fluid 26, as shownin FIG. 8, any of the fluid electrolyte mixtures described above, suchas aqueous solutions of sulfuric acid or potassium hydroxide or ammoniumsalts dissolved in glycol may be employed as the electrolyte. The choiceof the electrolyte, as in the other capacitor embodiments, depends uponthe composition of other materials employed in the capacitor. Thematerials chosen must be compatible so that no element is undulyattacked by another material that is present, thereby shortening thelife of the capacitor. When a liquid electrolyte is used, it isdesirable to include spacers 25 between the porous coatings 24 and theanode 6 to avoid direct contact. The same kinds of spacers as describedabove can be used, e.g., polymeric masses, NAFION films, or anotherinsulating material that resists attack by the electrolyte, in order tomaintain the desired spacing between the oxide coatings and the anode.Alternatively, the electrolyte can be a solid electrolyte 26', such as asolid mass of NAFION, polyaniline, or polypyrrole, which eliminates theneed for spacers 25. Preferably, the metal plates 21 are separated by aperipheral sealant 23 that also encloses the anode and the electrolyte.The plates 22 are electrically connected together as the cathode of thecapacitor and an anode connection is made by a wire passing through thesealant 23. As in other sectional views of embodiments of the inventiondescribed here, FIG. 8 does not indicate the geometry of the capacitorembodiment in a plan view. That plan view geometry can be any arbitraryshape, e.g., a circle, a rectangle, or a star shape, to fit a particularapplication. When a solid electrolyte is employed in the capacitorembodiment of FIG. 8, the sealant 23 is not necessary, at least inparticular applications of the capacitor.

Still another embodiment of a capacitor according to the invention isillustrated in a cross-sectional view in FIG. 9. That capacitor includesa unit cell 30'. Multiple unit capacitor cells 30' are stacked on eachother to form a capacitor with a cathode assembly at one end and ananode assembly at the other end. The capacitor cell 30' includes a metalfoil 22 on one side of which an electrically insulating oxide film 22'is disposed and on the other side of which a porous coating 24, of thetype previously described herein including at least one metal oxide isdisposed as a cathode. A solid electrolyte 26' is in contact with theporous coating 24 opposite the metal foil 22. The solid electrolyte maybe any of the electrolytes previously discussed here, includingpolypyrrole, NAFION, and polyaniline as well as other suitable solidelectrolytes. Most preferably, the metal plate 22 is a thin aluminumfoil having an appropriate configuration to provide a large surfacearea. For example, a preferred foil is a high etch ratio aluminum foilin which the effective surface area is increased by chemical treatment,for example, by 30 to 50 and even 100 times as compared to the projectedarea of the foil. Such foils are available from Kawatake ElectronicsCo., Ltd., Tokyo, Japan. The oxide film on the plate or foil 22' can bereadily formed by conventional techniques, such as anodic oxidation ofthe aluminum film.

In practice, a number of the unit cells 30' are manufactured and thenlaminated in a stack to form a capacitor body. At the end of the stackterminating an electrically insulating oxide layer 22', a cathodestructure including a metal foil 21, such as the high etch ratioaluminum foil, a porous coating 24 forming a cathode disposed on thefoil, and a solid electrolyte 26' opposite the aluminum foil 21 of aunit cell 30' is arranged. A cathode lead 2 extends from the aluminumfoil 20 of the cathode structure. The electrolyte 26' is in contact withthe electrically insulating oxide 22' exposed at the end of the stack ofunit cells. At the opposite end of the stack another metal plate or foil22' bearing an oxide is arranged. The oxide of that anode structure isin contact with the solid electrolyte 26' that is exposed at the end ofthe stack. An anode lead 4 extends from the aluminum foil 22 of theanode structure. Together, the unit cells and the cathode and anodeassemblies form a capacitor that has a variable capacitance dependingupon a number of unit cells 30' that are included in the laminatedstack.

The invention has been described with respect to certain preferredembodiments. Various additions and modifications within the spirit ofthe invention will be apparent to those of skill in the relevant arts.Accordingly, the scope of the invention is limited solely by thefollowing claims.

I claim:
 1. A capacitor comprising:a cathode comprising a porous coatingincluding an oxide of at least one metal selected from the groupconsisting of ruthenium, iridium, nickel, rhodium, platinum, palladium,and osmium; an anode spaced from the porous coating and including ametal selected from the group consisting of tantalum, aluminum, niobium,zirconium, and titanium; and a solid electrolyte disposed between and incontact with the porous coating and the anode and selected from thegroup consisting of polypyrrole, a polymer containing ion-permeablechannels, polyaniline, and a gel of an aqueous electrolyte and silicawherein the cathode comprises two opposed electrically conductingplates, each plate including the porous coating, the porous coatingsfacing each other, the anode is disposed between and spaced from each ofthe porous coatings, and the electrolyte is in contact with each of theporous coatings.
 2. The capacitor of claim 1 including a metal containercontaining the anode and electrolyte and on which the porous coating isdisposed.
 3. The capacitor of claim 1 wherein the anode is poroussintered tantalum having an oxide coating.
 4. The capacitor of claim 1wherein the anode is aluminum coated with an oxide of aluminum.
 5. Thecapacitor of claim 1 wherein the porous coating includes a mixture of atleast one oxide chosen from the group consisting of oxides of ruthenium,iridium, nickel, rhodium, platinum, palladium, and osmium and at leastone oxide chosen from the group consisting of oxides of tantalum,titanium, and zirconium.
 6. The capacitor of claim 1 wherein the porouscoating includes a mixture of oxides of ruthenium and tantalum.
 7. Acapacitor comprising:a cathode comprising a porous coating including anoxide of at least one metal selected from the group consisting ofruthenium, iridium, nickel, rhodium, platinum, palladium, and osmiumdisposed on each of two opposed electrically conducting plates; an anodedisposed between and spaced from the porous coatings and theelectrically conducting plates and including a metal selected from thegroup consisting of tantalum, aluminum, niobium, zirconium, andtitanium; and an electrolyte disposed between and in contact with theporous coatings and the anode.
 8. The capacitor of claim 7 wherein theelectrolyte is a fluid and including a sealant contacting theelectrically conducting plates and containing the electrolyte within thecapacitor.
 9. The capacitor of claim 8 including spacers maintaining aseparation between the anode and the porous coatings.
 10. The capacitorof claim 9 wherein the spacers are selected from the group consisting ofa glass fiber paper, plastic fibers, and an ion-permeable material. 11.The capacitor of claim 7 wherein the electrolyte is a solid electrolyteselected from the group consisting of polypyrrole, a polymer containingion-permeable channels, polyaniline, and a gel of an aqueous electrolyteand silica.
 12. The capacitor of claim 11 wherein the electrolyte is afluid.
 13. The capacitor of claim 12 wherein the electrolyte is sulfuricacid.
 14. A capacitor cell comprising:a first metal body; a cathodecomprising a porous coating including an oxide of at least one metalselected from the group consisting of ruthenium, iridium, nickel,rhodium, platinum, palladium, and osmium disposed on the first metalbody; a second metal body spaced from the porous coating; an anodeincluding a metal selected from the group consisting of tantalum,aluminum, niobium, zirconium, and titanium disposed on the second metalbody opposite the first metal body; and a fluid electrolyte disposedbetween and in contact with the porous coating and the anode.